1. Field of the Invention
The present invention relates to a magnetic levitating transportation apparatus including a vehicle capable of travelling along a rail which comprises a plurality of rail elements arranged in an end-to-end abutment relationship, and in particular, the present invention relates to a magnetic levitating transportation apparatus including a vehicle capable of stably passing a rail gap where the ends of two rail elements abut.
2. Description of the Related Art
In recent semiconductor manufacturing industries, wafers are often automatically transported in wafer treatment processes so that wafers are contaminated by dust as little as possible. A magnetic levitating transportation apparatus such as a linear motor vehicle is suitable for an automatic transportation apparatus for wafers because there is no friction between the vehicle and the rail and thus dust is not generated. In addition, a magnetic levitating transportation apparatus makes it possible to transport wafers quickly and reliably.
The magnetic levitating transportation apparatus includes a vehicle capable of levitating relative to a rail installed along a predetermined path with a gap between the vehicle and the rail. The vehicle travels along the rail. The vehicle is driven along the rail by a linear motor and caused to magnetically levitate by magnet units which are arranged on the vehicle. Also, at least one gap sensor is arranged on the vehicle for detecting the gap between the vehicle and the rail to control the current supplied to the magnet units to maintain the gap between the vehicle and the rail at a desired constant value. For example, when four magnet units are arranged on the vehicle, one gap sensor is arranged at the center of each magnet unit so that the distance between the gap sensor and the vehicle coincides with the distance between each magnet unit and the vehicle.
Usually, the rail comprises a plurality of rail elements arranged in an end-to-end abutment relationship, and there is a rail gap between two rail elements. The rail gap is not physically connected by welding or a joint member; however, the ends of two rail elements are abutted against each other with the rail gap therebetween. Accordingly, the rail gap is a discontinuous portion of the rail.
In the prior art magnetic levitating transportation apparatus, there is a problem that when the gap sensor approaches a rail gap, the gap sensor outputs an output value higher than an actual flight gap between the magnet unit and the rail. Accordingly, a large current is supplied to the magnet unit associated with the gap sensor passing through the rail gap, resulting in a loss of position of the vehicle. Then, after the gap sensor passes through the rail gap, the current supplied to the magnet unit is abruptly restored to a normal value, and a vibration may occur in the position of the vehicle.
This problem can be solved by eliminating the rail gap, but in order to eliminate a rail gap, a further problem arises in that it is necessary to ensure precise machining accuracy of the rail and to prepare an additional adjusting rail to compensate for an error, and it is difficult to lay the rail in the factory and to connect the rail elements precisely. Also, if a large current is supplied to the magnet unit when the vehicle passes the rail gap, the battery power is consumed in a short time and the charging time is increased, so that the availability of the apparatus is reduced.
Further, the magnet unit for causing the vehicle to levitate is designed to have a width in correspondence with the width of the rail to effect a self-centering action relative to the rail. That is, when an external force is applied to the vehicle and the magnet unit is transversely displaced from the rail, the magnet unit automatically returns to just below the rail by the action of the magnetic attraction between the magnet unit and the rail, and the vehicle can travel exactly along the rail.
However, it is preferable that the magnet unit have a rectangular cross-sectional shape rather than a square cross-sectional shape. In this case, if the vehicle passes crossing rails with the vehicle directed in a constant direction, the self-centering action varies. That is, assuming a first case in which the vehicle travels along a first rail portion of the crossing rails with the long side of the magnet unit directed perpendicular to the first rail portion, and a second case in which the vehicle travels along a second rail portion of the crossing rails with the short side of the magnet unit directed perpendicular to the second rail portion, and if a self-centering action is fully ensured in the first case, a self-centering action in the second case is not as fully ensured because the width of the magnet unit in the second case does not correspond to the width of the rail. Also, if the magnet unit is constructed to have a square cross-sectional shape, the total size of the magnet unit, and the weight of the vehicle, increases.